This invention relates to an improved parison separation assembly for use in blow molding machines, and more particularly, to a separation assembly that reduces the amount of polymer formed at the edge of the flash.
As will be appreciated by those skilled in the art, blow molding technology is widely used to manufacture hollow plastic articles, such as containers. A parison is formed by extruding a thermoplastic material and the parison is positioned between the separated halves of a mold. The mold halves are closed around the parison and a separation assembly separates the portion of the parison in the mold from the extruded parison outside the mold. Air is injected into the parison inside the mold to press the outer surface of the parison against the inner surface of the mold. When the object thusly formed cools, the mold is opened and the object is ejected from the mold. FIG. 1 illustrates a blow molding machine of the type used in the art, in which a series of molds are carried on a rotating wheel. In this machine, the parison is inserted from below into each mold when the mold reaches the nine o""clock position in the rotating cycle. The blow molded object cools as the wheel rotates in a clockwise direction. The mold opens, and the object is ejected, when it reaches the six o""clock position.
FIG. 2 illustrates a prior art parison separation apparatus used in a blow molding machine of the type shown in FIG. 1. On one half 12 of each mold a shear plate 14 is secured by machine screws 16 to an outside surface of the mold: On the other half 18 of the mold, a cut-off knife 20 is secured to a plate 22 which raises the cut-off knife so that its lower surface slides over and engages the shear plate 14 when the mold halves close. Screws 21 secure the cut-off knife 20 and the plate 22 to the mold half 18. As the two mold halves are moved together, the leading edge of the cut-off knife 20 contacts the plate 14, and as the mold halves continue to move together, the assembly separates the portion of the parison inside the mold from that outside the mold. While generally satisfactory, this separation assembly has several disadvantages, particularly when used to separate two or more parisons that feed two or more object forming cavities in a single mold. The prior art parison separation assembly is freely mounted on the edge of the mold and this assembly extends to cover the length between two or more parisons, which are each several inches apart. This prior art design, while working well for single parison separation, does not separate dual parison efficiently. One of the problems is the length of the knife assembly. To separate each parison satisfactorily, the knife""s specification must be tight beyond reasonable manufacturing expectations. This makes the prior art design expensive with a short life span, as it cannot be resharpened within the tight specifications.
Another problem with the prior art design is high temperature of the polymer at the end of the flash. It generates xe2x80x9cstringsxe2x80x9d which affect ejection. Also, some ejected items stick together as they move downstream and jam the conveyor and/or detabber. This problem is caused by polymer flowing into the open space 23 on both sides of each parison, leaving a mass of material at the edges of the flash, which retains heat.
An object of this invention is the provision of a parison separation assembly that does not leave stringers, and does not leave a bead of hot polymer attached to the flash.
Another object of the invention is the provision of a parison separation assembly that has a long operating life and provides good separation in multiple cavity, multiple parison blow molding machines.
Briefly, this invention contemplates the provision of a separate parison separation assembly for each parison, each with a shear plate secured in a recess machined into the mold in order to provide a positive and repeatable location of the shear plate. The shear plate is square and provides eight cutting edges; each of the four sides of the shear plate by rotating the shear plate as an edge wears, and by turning it over. The shear plate has a useful life up to eight times that of the prior art designs. The cut-off knife for each assembly is secured to the other half of the mold, directly on its surface, with a compression pocket cut into the face of this half of the mold to contain and to compress the parison between the mold halves. The design eliminates open space at the edge where the flash forms, reducing the mass of polymer at the edge and thus the temperature of the flash edge.